Poly(hydroxyalkanoic acid) (PHA) such as polylactic acid (PLA) is a resin comprising renewable monomer such as produced by bacterial fermentation of plant matter that include corn, sugar beets, or sweet potatoes. The resin can be used for transparent thermoformed packaging articles such as cups, trays, and clam shells. Generally, the resin is first extruded into an amorphous sheet, the sheet is then heated to above the glass transition temperature (Tg), and vacuum or pressure formed using an unheated mold into finished articles. The benefits of articles PHA are counter-balanced somewhat by dimensional instability of those articles at hot, ambient conditions.
Sections of the articles of low degrees of stretch such as less than about a 2.5-fold (250%) lengthening do not fully crystallize because many PLA grades crystallize too slowly in short cycle time thermoforming equipment or crystallize with less than 10% crystallinity in the given time. As PLA grades popular for thermoforming have a Tg of 55° C., articles of such PLA that are thermoformed into cool molds have poor dimensional stabilities when heated above the 55° C. Since packaging articles of PLA can experience temperatures slightly above 55° C. during transport or storage it is desirable to find ways to give PLA articles better mechanical stability under such conditions. A thermoformed or stretched article may shrink in a few seconds by more than 5% (sometimes 50%) when heated above the Tg. The tendency for shrinkage may be especially high (to 50%) in those parts of a molded article that experiences a critical amount of orientation between about 25% (final length or area is 25% greater than the pre-formed length or area) and about 100%. Those regions having higher than about 100% or about 250% orientation may experience some added crystallization during the forming process. Accordingly, they may have a lowered shrinkage tendency than other regions. Those areas having no orientation may have low shrinkages (<10%); however, these areas are soft and easily deform when mechanically stressed at temperatures slightly above Tg. Those regions in between about 100% and about 250% may have the highest shrinkage which is most benefited by the subject of this application. High forces can be generated by shrinkage and therefore the shrinkage of one region in a complex hollow article can be magnified into a larger dimensional effect on the overall structure. Therefore for the purpose of this application the desirable shrinkage is less than 8%, less than 4%, or less than 1%.
The shrinkage force can be due to the presence of stretched PLA molecules not crystallized but amorphous and frozen in place by the rapid cooling in the mold, termed “amorphous orientation”. When the temperature rises above Tg these molecules relax in a few seconds and induce or cause shrinkage if the article is not constrained from shrinking (i.e. shrinkage-by-relaxation). Some additional shrinkage in a few minutes can arise from crystallization (i.e. shrinkage-by-crystallization) if the PLA is not crystallized fully to its capacity and the temperature of the article is considerably above the Tg and especially at the average temperature of the Tg and melting point. A particularly fast crystallizing PLA due to its low molecular weight (such as below 10,000 g/mole), low D-lactide (meso-lactide) content, and/or use of high amounts of special nucleators may enable shrinkage-by-crystallization to happen below the average of Tg and the melt point if the article is not crystallized to the capacity of the PLA. The benefit of this application is for low haze.
To solve the shrinkage or dimensional stability problems, one may increase crystallinity and/or decrease amorphous orientation. A numerical ratio therefore to be minimized is the amount of amorphous orientation versus total crystallinity. Such a ratio, which can be defined by x-ray, is the ratio of amorphous orientation determined by x-ray to total crystallinity determined by x-ray and preferably is less than about 2 or less than about 1 or more preferably less than about 0.1. Another indication of amorphous orientation might be via correlation between x-ray and an endotherm just above Tg determined by Differential Scanning Calorimetry (DSC) termed “enthalpy relaxation”. Articles of PLA having amorphous orientation by x-ray may have about 9 J/g of an endotherm within 5° C. above Tg during 10° C./minute heating of samples. Samples without amorphous orientation have 0 J/g of this endotherm. Data suggests that it may not entirely focus on maximizing crystallinity for low shrinkage-by-relaxation, but may minimize the ratio of the endotherm at Tg to total crystallinity to less than about 0.3, or 0.1 or 0.05. Minimizing the ratio of amorphous orientation to total crystallinity instead of maximizing total crystallinity may alleviate high levels of crystallinity which tend to introduce higher levels of haze in conventional thermoforming processes.
There may be methods for minimizing the ratio of amorphous orientation to total crystallinity. For example, to increase the crystallinity of a PHA or PLA having a Tg of 55° C. and a melt point of 155° C., one may heat-treat the finished molded article at 105° C. for several minutes to give the article hardness when heated above 55° C. However, doing so may cause the article to shrink in the first few seconds of the heat treatment.
One may heat-treat the article for several seconds at about 105° C. in a heated mold while it is constrained from shrinking in that mold. Doing so would leave as amorphous those regions of the article that have not been oriented more than about 25%. Removal of the article from the hot mold would cause deformation of those regions possibly due their sticking to the hot mold or their deformation under low mechanical stresses while still hot.
One may heat-treat the article for several minutes at about 105° C. while it is constrained from moving in the heated mold. The removal process would not deform the article since it is hardened by crystallinity. Doing so may extend the thermoforming cycle time to an inefficiently long time.
Alternatively, one may reduce the original amount of amorphous orientation, by molding an article at a high temperature, above the average of Tg and melt point. Excessively high temperatures, more than about 25° C. such as approaching the melt point, may give excessive sagging of the hot sheet or deformation at its supports before being thermoformed. Slightly lower excessive temperatures could be problematical due to exudation of oligomer or additives on the surface of the mold giving surface roughness to the molded article. Running at high temperatures (e.g., at about 25° C. above the average of Tg and melt point) may give a molded article having few stretched amorphous PLA molecules and reduced shrinkage compared with a molded article having stretched amorphous molecules. However, the article may be 90% or more amorphous, which may be soft and easily, undesirably deformable in its use above the Tg, while an article of >10% crystalline is generally desired for its being dimensionally stable above its Tg. A 90% amorphous article may also experience some shrinkage when held for weeks at temperatures above 55° C. due to some beginnings of crystallization or other molecular re-arrangement.
To increase the crystallinity, one may mold an article at high temperature and anneal the article in the molds at a temperature of the average of Tg and the melt point to increase the crystallinity but doing so may greatly increase the haziness of the article.
Alternatively an article can be made such that the resin is stretched during thermoforming to more than about 150% and heat treated for a few seconds at the average of Tg and melting point. Doing so may give clarity and dimensional stability due to strain-induced crystallization (that is, crystallization during the stretching process), but this large amount of stretching limits the shape of molded articles to those that are very long and narrow.
One may also increase the crystallinity or rate of crystallization by use of a nucleator for PLA. Nucleators include talc, calcium silicate, sodium benzoate, calcium titanate, boron nitride, copper phthalocyanine, and isotactic polypropylene. Using nucleator introduces high haze or opacity to the otherwise transparent PLA articles thereby impairing the value of the articles. See, e.g., U.S. Pat. No. 6,114,495, U.S. Pat. No. 6,417,294, and WO 03014224.
Therefore, there is a need to produce a low haze (i.e., contact clarity, which means substantially clear to read through by human eyes) article from PHA and to increase the dimensional stability throughout the surface of the low haze article. We have found that very hot molds may be used with PHA to generate 0% shrinkage-by-relaxation above Tg, that nucleators allow the use of such hot molds and that crystallizing PHA to their maximum capacity is not necessary for such dimensional stability.